1. Field of the Invention
The invention relates to image sensor device fabrication, and more particularly relates to a method for fabricating an image sensor device capable of light illumination from a back side of a substrate therein.
2. Description of the Related Art
An image sensor includes a grid of pixels comprising elements such as photodiodes, reset transistors, source follower transistors, pinned layer photodiodes, and/or transfer transistors for recording an intensity or brightness of an incident light. Each of the pixels responds to the incident light by accumulating a charge. The more incident light, the higher the charge. The charge can then be used by another circuit so that a color and brightness can be used for a suitable application, such as a digital camera. Common types of pixel grids include a charge couple device (CCD) or complimentary metal oxide semiconductor (CMOS) image sensor.
Typically, an image sensor is a semiconductor device converting an optical image to an electrical signal. Among the above mentioned image sensors, the CCD image sensor comprises a plurality of metal-oxide-silicon (MOS) capacitors for storage and transfer of charge carriers. The CMOS image sensor, a product of CMOS manufacturing technology, is a semiconductor device that converts an optical image to an electrical signal using a switching scheme of a MOS transistor for transportation of photo-electric charges from a photodiode to an output node as well as detection of an output signal at the output node.
FIG. 1 is a cross section showing a CMOS image sensor having a plurality of microlenses therein as disclosed in U.S. Pat. No. 6,979,588 (issued to Jeong et al.). As shown in FIG. 1, the CMOS image sensor includes a semiconductor substrate 10 with a plurality of isolation regions 12 formed therein, defining a plurality of pixel regions. A photodiode 14 for converting an incident light to photo-charges is formed in a corresponding pixel region. For the sake of convenience, transistors required for the unit pixel is not depicted in the drawings. An interlayer dielectric (ILD) 16 is formed on the semiconductor substrate 10, thereby metal interconnects 18 are provided at predetermined locations of the ILD 16 to prevent shielding of incident light on the underlying photodiodes 14. A passivation layer 20 is formed over the metal interconnects 18 for protecting a device from moisture and scratching during post-manufacturing processes. In addition, color filter array 22 having red, green and blue color filters are formed directly on the passivation layer 20. An over-coating layer (OCL) 24 is providing on the color filter array 22, thereby providing a planarized surface. A plurality of dome shaped microlens 26 is formed on the OCL 24, substantially corresponding to each of the pixel regions. Therefore, incident light 30 projected onto the photodiodes 14 can pass through the internal structures formed between the photodiodes 14 and the microlenses 26 along an optical path L1.
Nevertheless, with the trend toward size reduction of pixel units formed on a substrate, optical interference such as refraction, reflection, diffraction and light absorption may occur during the progress of the incident light 30 along the optical path L1 by the metal interconnect structures and the materials formed therein, thereby affecting the quantum efficiency of the photodiodes 14 and cross-talk may thus occur between adjacent pixel units.